The present invention relates to a device for the local attenuation of the light intensity in the field of vision of a light-sensitive monitoring and/or recording device, for use, for example, as a glare protection device for video cameras in videophones or correspondingly equipped PCs.
The enormous progress of modern telecommunications technology will soon make interactive audiovisual communications between two and more persons via videophone or PC a part of our everyday life. In addition to a speaker and a microphone, the terminal devices used for this purpose also include a video camera for recording the persons communicating with each other as well as a screen for displaying them. They are generally located in homes or offices that are used for diverse purposes and have variable lighting which changes over time.
In this connection, the video cameras used are aimed simultaneously at the person telephoning as well as his/her background with possibly extremely variable lighting, which may possibly include even very bright light sources such as the sun or a halogen lamp. Of course, they are controlled in this manner so that the image of the person telephoning is optimally recorded and transmitted to the screen of the other party to the conversation. The dynamic range of the video camera is, however, not great enough to image the bright backgrounds or light sources with optically sharp definition. This results in bloom, which diminishes the quality of the image of the person telephoning.
An automatic reduction of sensitivity, for example, proves to be inadequate for solving this problem, since the image of the person telephoning then becomes excessively dark. For that reason, special non-linear electro-optical components have been used recently, which are locally transparent with weak to average lighting and are locally opaque with strong or very strong lighting so that bright light sources are more strongly attenuated than dark ones. Consequently, local light sources in an image are blended out without darkening the rest of the image.
These components are optically addressable spatial light modulators based on liquid crystals as described, for example, by M. G. Tomilin, A. P. Onokkow and B. Y. Polushkin in Mol. Cryst. Lq. Crys., 222, 119 (1992) on the basis of safety glasses. In this connection, the twist effect in nematic liquid crystals is utilized as the medium acting on the light. However, the principal disadvantage in using nematic liquid crystals is the relatively slow switching time, which is in the range of approximately 10xe2x88x922 sec. This leads to blurring or smudging of the suppressed image during the image movement of bright objects in the light modulators used or to the glare of the eye or of a video camera used.
German Patent Application No. 196 16 323.4 and related U.S. patent application Ser. No. 08/838,965 by L. Beresnev et al. describe a high resolution ferroelectric liquid crystal modulator having a substantially higher operating frequency of 102-103 Hz, the modulator being based on helical smectic liquid crystals in contact with an upstream semi-transparent photo-conductive layer rather than nematic ones (see FIG. 1 of the present application and the associated part of the description). The photo-conductive layer is arranged in the image plane of an imaging objective so that the images of the objects located in the field of vision of the objective are converted into voltage patterns which are transmitted to the liquid crystals and correspondingly reorient them. The liquid crystal modulator arranged between two polarizers thus becomes locally transparent for low light intensities and locally darkened for high light intensities so that bright light sources or backgrounds are locally suppressed without the occurrence of glare or bloom of the image.
The practical application of this liquid crystal modulator is described in the exemplary embodiments only on the basis of glasses in which the images arising on the photo-conductive layer in the most uncomplicated case are first imaged by a lens in such a way that they can be observed by the eye via an additional lens acting as a magnifying lens, the lens being selected and arranged in such a way that a virtual enlarged erect image of the second intermediate imaging comes into being. The described liquid crystal modulator can be made optically inoperative if, in certain states of the applied voltage cycles, it does not have the desired glare-reducing properties.
An object of the present invention is to produce an improved device based on such a ferroelectric liquid crystal modulator for the local attenuation of the light intensity in the field of vision of a light-sensitive monitoring and/or recording device, the device being suitable in particular as a glare protection device for video cameras in videophones or correspondingly equipped PCs.
The present invention provides for the intermediate imaging of the objects located in the field of vision which arises in a device for the local attenuation of the light intensity in the field of vision of a light-sensitive monitoring and/or recording device (CCD) comprising a first objective (L1) made up of one or several lenses and a high resolution ferroelectric liquid crystal modulator (PC, LC) arranged between two polarizers (P1, P2), the liquid crystal modulator including a photo-conductive layer (PC) and a downstream liquid crystal cell (LC) having helical smectic liquid crystals which is in contact with this layer and is arranged in such a way that the photo-conductive layer (PC) is located in the image plane of the first objective.
The present invention is characterized in that the intermediate imaging is imaged directly onto the monitoring and/or recording device from the photo-conductive layer of the liquid crystal modulator by a second objective made up of one or several lenses so that the second intermediate imaging additionally required in the related art is eliminated, which results in a more compact, lighter and more cost-effective glare protection device.
A second possible solution is to omit the intermediate imaging and to image the objects located in the field of vision directly onto the light-sensitive monitoring and/or recording device by an objective made up of one or several lenses, the monitoring and/or recording device accordingly being essentially arranged in the image plane of the objective. The high resolution ferroelectric liquid crystal modulator is in this case arranged in close proximity in front of the monitoring and/or recording device in the beam path of the objective, the liquid crystal modulator being located in turn between two polarizers. An optical fiber plate may be arranged between the high resolution liquid crystal modulator and the monitoring and/or recording device for improved image transmission. The present invention thus also provides a device for the local attenuation of the light intensity in the field of vision of a light-sensitive monitoring and/or recording device (CCD) comprising an objective (L1) made up of one or several lenses and a high resolution ferroelectric liquid crystal modulator (PC, LC) arranged between two polarizers (P1, P2) in the beam path of the objective, the liquid crystal modulator including a photo-conductive layer (PC) and a downstream liquid crystal cell (LC) having helical smectic liquid crystals which is in contact with this layer, characterized in that the monitoring and/or recording device (CCD) with an upstream liquid crystal modulator (PC, LC) is arranged in the image plane of the objective (L1).
In this type of glare protection device, the objects are naturally imaged somewhat indistinctly on the photo-conductive layer of the liquid crystal modulator so that bright interference light sources are advantageously shown not only darkened but also blurred. In addition, omitting the intermediate imaging also makes it possible to substantially reduce the overall length and weight, since the second objective is eliminated without being replaced. Since this objective is customarily of high quality and expense, this embodiment of the present invention is also connected with considerable cost savings.
Since the light incident on the photo-conductive layer of the liquid crystal modulator is partially absorbed, both embodiments preferably include a color filter via which the light""s spectral components are changed in such a way that the light-sensitive monitoring and/or recording device, which is preferably a CCD camera, delivers an image of the person telephoning or of another object located in the field of vision having color fidelity. The color filter preferably located between the first objective and the photo-conductive layer thus acts as a color correction filter for the components of the camera and for the color distortion caused by the color temperature of the light sources and must be tuned accordingly.
In color cameras, a mosaic color filter is preferably arranged directly in front of the light-sensitive monitoring and/or recording device.
In addition, the described devices preferably include an optical shutter or compensator to interrupt the beam path if the liquid crystal modulator does not have the desired glare-reducing properties in order to prevent possible damage to the light-sensitive monitoring and/or recording device as a result of excessively high light intensities. In this connection, the optical shutter may be designed as a fast ferroelectric liquid crystal switch as described, for example, in the above-mentioned German Patent Application 196 16 323.4 by Beresnev et al. However, it may also be designed as a different electro-optical or electromechanical shutter or switch such as are known to experts in this field. For example, an electro-optical shutter designed as a switchable retardation plate is arranged between two polarizers in order to obtain an extinguishing of light. According to the present invention, the shutter can be arranged both between the front objective and the liquid crystal modulator and between the liquid crystal modulator and the light-sensitive monitoring and/or recording device.